The present invention relates to method for producing a shadow mask for a colour cathode ray tube, and more particularly relates to an improvement in production of a shadow mask for a colour cathode ray tube from spinodal decomposition type magnet alloys.
It is basically required for a shadow mask for a colour picture tube to exhibit high image discrimination with high brightness.
In order to meet this requirement, the U.S. Pat. No. 4,135,111 proposes an after-focusing type cathode ray tube. In the construction of this prior proposal, a magnetic field is generated in an electronic beam passage through a shadow mask in order to promote focusing of electronic beams and enrich the rate of passage of the electronic beams through the shadow mask, thereby obtaining high brightness. In order to enable generation of such magnetic field, the shadow mask is made of magnetic materials such as Cu-Ni-Co alloys or Cu-Ni-Fe alloys. However, the rate of thermal expansion of such magnetic material is in general very high. For example, it amounts to about 14.times.10.sup.-6 /.degree.C. Such high rate of thermal expansion allows undesirable doming due to thermal deformation of the shadow mask, thereby seriously degrading colour purity of the image obtained.
As a substitute for such highly expandable alloys, the use of spinodal decomposition type magnet alloys of lower rate of thermal expansion has already been employed in the field. Such materials have excellent workability and their rate of thermal expansion is about 10.times.10.sup.-6 /.degree.C.
In conventional production of a shadow mask from spinodal decomposition type magnet alloys, the material is first shaped into a curved configuration of the shadow mask and spinodal decomposition is effected after aging. During this process, strain evolved during heat treatment causes deformation of the original configuration. As a consequence, the electronic beam passage is biased from the correct position, thereby causing undesirable colour slip. Thus, the conventional process failed to produce shadow masks well suited to actual use.